The present invention concerns a watch crystal including an optical lens shaped in the thickness of the material of the crystal, as well as a manufacturing method for such a lens.
Making a magnifying lens on a watch crystal in order to facilitate reading of a small portion of the display means of the watch, for example a date indicator in a window, is well known. According to a common embodiment, the lens projects on the upper face or the lower face of the crystal, the lens being able to have any shape. The lens can then be made integral with the crystal if the latter is manufactured by moulding, for example of a synthetic material such as PMMA. But if the crystal is made of a hard material such as sapphire, which is difficult to machine and polish, a projecting lens of this type is generally manufactured separately, then bonded onto the crystal. This presents assembly difficulties and the risk of the lens becoming detached with use. Moreover, projecting lenses are exposed to wear and to shocks if they are on the outer face of the crystal, or they represent an inconvenient bulk as regards the hands of the watch if they are on the inner face.
This has led to making lenses arranged in the thickness of the crystal, preferably in the lower face in order to avoid dirt being deposited along the edge thereof. European Patent No. 0 123 891 discloses a method for forming such a lens in a plate made of a mineral material, in particular sapphire, by machining by means of a rotating grinding wheel with an oscillating movement of the wheel or the plate. In practice, this method is only suitable for manufacturing circular lenses.
Another construction, disclosed in the European Patent Publication No. 0 814 388 A, consists in manufacturing separately the lens and the plate intended to form the crystal, arranging in the thickness of the plate a blind recess having a depth substantially equal to the thickness of the lens, then fixing the lens in this recess. The plate and the lens can in particular be made of glass, spinel, corundum or sapphire. This offers the advantage of allowing the lens a non-circular shape, for example an elongated shape to facilitate reading of an indication in a rectangular window. However, the manufacturing method remains quite complicated, in particular because the recess has to be machined and polished as well as the two faces of the lens, and the risk of the lens becoming detached remains.
An object of the present invention is to avoid the aforementioned drawbacks, in particular with crystals and lenses made of hard material, by providing a simple, resistant and attractive watch crystal.
A basic idea of the invention consists in making a watch crystal of corundum, sapphire or spinel, including an optical lens shaped in the thickness of the crystal material, this lens having a non-circular contour. In a preferred embodiment, the contour can have reentering angles and can for example be rectangular.
To our knowledge, the fact that such a watch crystal has not been made before results from the fact that one did not know how to manufacture it. Indeed, if a method had been found for machining a recess in a plate of hard material such as sapphire and one could envisage giving the bottom of this recess the convex shape of the lens, a mat surface would be obtained and it was not known how to polish it to give it the desired optical and aesthetic properties. This problem is resolved with the method which will be described hereinafter.
More particularly, another aspect of the invention concerns a manufacturing method for a non-circular optical lens in the thickness of a watch crystal made of a hard mineral material, in particular corundum, sapphire or spinel, characterised in that it includes the successive steps of forming by machining in one face of the crystal a recess of non-circular contour, the recess having lateral walls and a convex bottom intended to constitute a surface of the lens, and polishing at least the bottom of the recess by means of a brush having a reciprocating movement to hit the bottom, with the addition of a polishing medium.
The machining can be effected for example by ultrasound by means of a sonotrode, or by engraving by means of a rotating tool. In the polishing step, the brush can be animated, in addition to its back-and-forth movement, by a transverse movement, for example as a result of a rotation about an axis substantially perpendicular to the crystal and situated at a distance from the lens.